A carburetor of a gasoline engine is mainly constituted by two parts of a mixing system and a float system. The float system is mainly constituted by a float chamber body, a float assembly, a float needle valve, a float chamber cap and so on. The mixing system is mainly constituted by a main nozzle, a venturi body, a main jet, a venturi, a throttle, a choke, an idle adjusting screw and so on. Moreover, the carburetor is further provided with an idling speed oil supply device: As the engine idles at low speed, the throttle is almost completely closed. However, the suction force is very large behind the throttle where an idle port communicates with the venturi with the oil ejection quantity of the idle port being controlled by the idle adjusting screw. Before leaving the factory, the idle adjusting screw has been adjusted to a proper position, thus ensures the normal operation of the gasoline engine under idle operating conditions and limits the exhaust-gas discharge of the gasoline engine to a minimum, thereby meets the requirements of the environmental protection. The idle screw will not change its position by itself under the spring force of a retaining spring. After the carburetor is obtained by the user, there is usually no need to readjust the idle adjusting screw. However, there exists the problem that it is unable to ensure the user not to readjust the idle adjusting screw. To prevent the user from adjusting the idle adjusting screw by himself during the using process which affects the discharge of the gasoline engine, it is desirable to limit the readjustment applied on the idle adjusting screw by the user.